Dehydration of α-methylbenzyl alcohols to form monovinylidene aromatic monomers

ABSTRACT

α-Alkylbenzyl alcohol and substituted α-alkylbenzyl alcohols are converted in high yield and purity to styrene and substituted styrenes by contacting the alcohol in vapor phase with silica gel and steam.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of our previousapplication Ser. No. 39,889 filed May 22, 1970 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the vapor phase dehydration of α-alkylbenzylalcohols and substituted analogues thereof to form styrene andsubstituted styrenes.

Dehydration of alcohols to their corresponding unsaturated structuralcompounds is well known in the art. Dehydration techniques are notgenerally employed in the manufacture of styrene and many homologuesthereof because standard dehydrogenation of ethyl benzene is consideredto be a more economic route. In addition, styrenes produced byconventional dehydration techniques often contain enough ethyl benzeneand other impurities to require extensive purification.

It is characteristic of standard dehydrogenation techniques employed inthe production of styrene that fairly large quantities of unreactedethyl benzene be present in the styrene fraction. Such quantities ofethyl benzene in the styrene fraction are substantial enough to causeloss of properties in polymers of such styrene fractions. Furthermore,due to the closeness of the boiling points of styrene and ethyl benzene,removal of ethyl benzene by distillation is expensive.

Moreover, normal dehydrogenation of many substituted ethyl benzenes,particularly the tertiary alkyl substituted ethyl benzenes, destroys oralters the substituted group. For example, dehydrogenation ofar-(t-alkyl)-ethyl benzene to form their corresponding styrenes usuallyresults in rupture and/or loss of the t-alkyl group as well as ethylgroup dehydrogenation.

Attempts to prepare ar-(t-alkyl)styrenes by conventional dehydration ofthe corresponding ar-(t-alkyl)-α-methylbenzyl alcohols have not beensatisfactory due to the formation of other byproducts and the rupture ofthe t-alkyl group which frequently accompanies dehydration. As a resultof this rupture, appreciable quantities of ethyl benzene and undercertain conditions, diolefinically unsaturated aromatic monomers areformed in addition to the desired ar-(t-alkyl)styrene. Thesediolefinically unsaturated aromatic monomers, e.g.,ar-(i-propenyl)styrene in dehydration of ar-(t-butyl)-α-methylbenzylalcohol, are very difficult to separate from the desiredar-(t-alkyl)styrene. During polymerization of the ar-(t-alkyl)styrenemonomer, the diolefinically unsaturated aromatic monomer acts as acrosslinking agent thereby producing a substantially crosslinked styrenepolymer which is insoluble in many organic solvents such as toluene andbenzene. This lack of solubility is undesirable in many applicationsemploying such styrene polymers.

Conventional dehydration techniques for preparing styrene andsubstituted styrenes are not completely satisfactory in that substantialamounts of ethyl benzene and other difficult to separate impuritiesoften remain or are produced. Such difficulties have been pointed out inprior publications such as U.S. Pat. Nos. 2,399,395 and 3,442,963.

Therefore, it would be highly desirable to provide a new, improvedtechnique for producing styrene and substituted styrenes in high yieldwhich contain little or no ethyl benzene and other impurities,particularly diolefinically unsaturated aromatic monomers.

SUMMARY OF THE INVENTION

Accordingly the present invention is an improved process for dehydratingα-alkylbenzyl alcohols and to form the corresponding styrene monomer inhigh yield and purity. This improved process comprises contacting anα-alkylbenzyl alcohol, as hereinafter described in detail, in vaporphase with a silica gel dehydration catalyst in the presence of fromabout 0.03 to about 25 parts by weight of added water per part by weightof alcohol.

Prior art teachings indicate that water produced during dehydration ofan alcohol should be removed from the reaction mixture in order to movethe reversible dehydration reaction to the right and thereby increasethe yield of the unsaturated product. Surprisingly in the process ofthis invention wherein silica gel is employed as the dehydrationcatalyst, it is found that presence of water, preferably accomplished byaddition of from about 0.03 to about 25 parts by weight per part ofalcohol to the alcohol prior to dehydration and/or during dehydration,effectively increases the yield of the desired styrene and substantiallyreduces, and under optimum conditions, almost completely eliminates theformation of ethyl benzene and other impurities which are difficult toseparate. In general, the desired monovinylidene aromatic monomer isproduced in purity greater than about 99 mole percent and contains lessthan about 0.5 mole percent of alkyl benzene impurity, so-called ethylbenzene impurity.

As a result, styrene monomers produced by this method require little orno further purification to remove impurities having boiling pointsnearly the same as the monomer; thus expensive distillation proceduresare eliminated. Styrene polymers produced from these styrene monomersare found to have improved properties as a result of the increasedpurity. As a result of low concentrations, i.e., less than 0.02 molepercent based on total monomer, of diolefinic impurity,ar-(t-alkyl)styrenes produced by this process can be polymerizeddirectly into polymers which are soluble in toluene, benzene and otherorganic solvents. Such organic soluble polymers are particularly usefulin various coatings, thermoplastic molding, reactive diluents, polyestervarnish and chemical applications that require a monomer essentiallyfree of diolefinic species.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of this invention, the term "α-alkylbenzyl alcohol"includes α-alkylbenzyl alcohols, especially α-methylbenzyl alcohol andsubstituted analogues thereof. Such alcohols are represented by thegeneral formula: ##STR1## wherein R is hydrogen, alkyl having from 1 to12 carbon atoms, e.g., methyl, t-butyl, t-amyl and other t-alkyl;halogen, e.g., bromo, chloro, and fluoro; or the like and R₁ is hydrogenor alkyl having from 1 to 4 carbon atoms.

Exemplary α-alkylbenzyl alcohols include α-methylbenzyl alcohol,ar-chloro-α-methylbenzyl alcohol, ar-bromo-α-methylbenzyl alcohol,ar-fluoro-α-methylbenzyl alcohol, ar-dichloro-α-methylbenzyl alcohol,ar-dibromo-ar-chloro-α-methylbenzyl alcohol, ar-chloro-α-ethylbenzylalcohol, 4-chloro-2,5-difluoro-α-methylbenzyl alcohol,ar-(t-butyl)-α-methylbenzyl alcohol, ar-(t-amyl)-α-methylbenzyl alcohol,ar,α-dimethylbenzyl alcohol, α-ethyl-2-isopropyl-5-methylbenzyl alcohol,α-isobutyl-2,4,5-trimethylbenzyl alcohol, and the like.

Preferred α-alkyl benzyl alcohols are αmethyl-benzyl alcohol,ar-halo-α-methylbenzyl alcohol such as ar-chloro- andar-bromoα-methylbenzyl alcohol and ar-(t-alkyl)α-methylbenzyl alcoholssuch as p-(t-butyl)α-methylbenzyl alcohol, p-(t-amyl)-α-methylbenzylalcohol and similar alcohols wherein t-alkyl has 4 to 8 carbon atoms.The above alcohols are known compounds and can be prepared by synthesesobvious to those skilled in the art. Illustratively,ar-alkyl-α-methylbenzyl alcohols can be prepared by the stepwisesynthesis of (1) alkylating ethyl benzene with olefin in the presence ofsulfuric acid in accordance with the method of Ipatieff et al., JACS,Vol. 58, 919(1936), (2) oxidizing the alkylated ethyl benzene to thecorresponding acetophenone-alcohol mixture as described by H. J. Sanderset al., I & E Chem, Vol 45, 2 (1953), and (3) reducing the mixture bycatalytic hydrogenation to the desired alcohol.

Silica gel dehydration catalyst employed in this invention may be in anyof the several forms of silica gel which will permit intimate contactbetween the silica gel and alcohol vapor during the dehydration. It isdesirable that the silica gel be in the form of a divided solid,preferably in the form of particles not measuring more than about aninch in any dimension. Further the silica gel should be of a type thatis not degraded or destroyed when contacted with large quantities ofwater. Although good results are obtained with a number of grades ofparticulate silica gel, best results are obtained with the silica gel inthe form of a particulate solid having a mesh size ranging from about 2to about 400 and a surface area of at least about 300 square meters pergram, preferably from 300 to 900 m² /g. In these preferred embodiments,the possibility of complete contact between the silica gel and thealcohol is maximized. It is especially preferred that the silica gel befinely divided porous particles having an average pore diameter rangingfrom about 2 to about 200 Angstrom Units. Methods for preparing silicagel are well known to skilled artisans. Also any of several commercialgrades of silica gel fitting the above general description may beemployed.

It is to be understood that the silica gel may be employed incombination with other materials such as binders, fillers and commonlyemployed inert supporting materials such as fused alumina, crushedsandstone, and silica, filter stone and ceramically bonded silica.

In the practice of this invention the α-alkylbenzyl alcohol in vaporphase is contacted with the silica gel in the presence of from about0.03 to about 25 parts by weight of water per part by weight of alcohol,preferably from about 0.5 to about 20 weight parts, especially fromabout 1 to about 2 parts of water per weight part of alcohol. It isgenerally preferable that the alcohol be intimately mixed with specifiedamounts of water in the form of steam prior to dehydration. This iseasily accomplished by passing liquid or vaporous mixtures of thealcohol and water over or through a bed or column of an effective heattransfer material such as silicon carbide, fused ceramic packing ornon-corrosive metal packing. In such embodiments, a column having alower portion of a bed of catalytic silica gel and an upper portion ofthe heat transfer agent can be made and the alcohol containing water isthen passed downward into the column through the heat transfer agent andthen through the catalytic silica gel bed. It is often desired to employan organic carrier liquid which is a solvent for the alcohol, e.g.,toluene or benzene, but which can be easily removed by simpledistillation. In such embodiments, the alcohol and carrier liquid aremixed together prior to vaporization of the alcohol mixture. It isunderstood addition of water to the reaction may be made after thealcohol has passed through the heat transfer agent. Also the water neednot be added in the form of steam or super-heated steam although it ispreferred to do so.

Generally, the desirable temperatures of operation of the process ofthis invention are in the range of about 200° to 510° C., preferablyfrom about 260° to about 450° C., especially from about 300° to about400° C. In the dehydration of ar-(t-alkyl)-α-methylbenzyl alcohols, itis desirable to employ dehydration temperatures above 260° C.,preferably about 325° to about 425° C., in order to insure contactbetween the dehydration catalyst and the alcohol in the vapor state. Itis generally desirable to carry out dehydration at atmospheric pressure,although it is possible to achieve dehydration with relatively goodpurity and yield at subatmospheric to superatmospheric pressure, e.g.,from about 0.2 to about 5 atmospheres. Vaporization of the alcohol,however, may be advantageously achieved by using reduced pressure.Vaporization may also be achieved by contacting the alcohol with steamor superheated steam substantially prior to dehydration.

The quantity of silica gel which effectively dehydrates the alcoholdepends in part upon the rate at which the vaporous alcohol is to bepassed through the silica gel bed or column, upon the surface area ofthe gel per unit of weight, upon the amount of water to be employed.Generally higher vapor flow rates and larger quantities of water requiremore silica gel to achieve effective dehydration.

Practice of the present invention as described hereinbefore yields thedesired monovinylidene aromatic monomer, particularly thear-(t-alkyl)styrene, in purity greater than 99 mole percent based ontotal product after simple distillation which removes unreacted ketonesand alcohols. Accordingly, the alkyl benzene impurity is held below 0.5mole percent. In the dehydration of the ar-(t-alkyl)-α-methylbenzylalcohols by the method of this invention, diolefinic and otherpolyolefinic impurity is held below 0.02 mole percent based on totalproduct after simple distillation.

The invention is further illustrated by the following examples whichshould not be construed as limiting the scope of the invention. Allparts and percentages are by weight unless otherwise indicated.

EXAMPLE 1

A mixture of 50 parts of α-methylbenzyl alcohol and 50 parts of tolueneis preheated to 300° C. and mixed with 100 parts of steam at 300° C. Themixture is passed downward through a glass column (1 inch outsidediameter × 27 inch length) equipped with an electric furnace andcontaining a 14-inch upper layer of silicon carbide (8 mesh) preheatedto 350° C. and a 6-inch lower layer (20 g) of silica gel (on 10 mesh,300 m² of surface area/g). Water and dehydrated organic product arecondensed in the lower part of the column, collected and separated. Theorganic product is dried and distilled. The distilled product isdetermined by infrared spectroscopy and vapor phase chromatography to be99⁺ mole percent styrene containing less than 0.5 mole percent of ethylbenzene. Overall yield on the basis of starting alcohol is greater than95 percent.

EXAMPLE 2

A mixture of 50 parts of 4-(t-butyl)-α-methylbenzyl alcohol and 50 partsof toluene is prepared. A reaction column (1 inch outside diameter × 27inch length) is filled to a bed height of 8-9 inches with silica gel(8-10 mesh, 340 m² of surface area/g, 140A average pore diameter) andsufficient amount of silicon carbide (6 mesh) is added to the tube toincrease total bed height to 16 inches. The reaction column is heated to300° C. Water preheated to 300° C. and the mixture are addedsimultaneously into the feed end of the column at rates of 90 ml/hr and45 ml/hr respectively. An intimate admixture of steam and the alcoholmixture in vapor phase is formed and passes downward through the siliconcarbide preheated to 350° C. which acts as a preheat section for thevapor and then through the silica gel to effect dehydration. Followingpassage through the silica gel, water and organic product are condensedin the column, and collected. The dehydrated organic product isdecanted, dried and distilled. The distilled product is determined byinfrared spectroscopy and vapor phase chromatography to be4-(t-butyl)styrene at 99 percent or greater purity. Overall yield onbasis of amount of starting alcohol is greater than 90 percent.

Polymerization of the 4-(t-butyl)styrene by heating in the presence ofbenzoyl peroxide yields a polymer which is soluble in toluene at 20° C.

EXAMPLE 3

Several sample runs are carried out generally according to the procedureof Example 2. In these runs, mixtures of 50 parts of4-(t-butyl)-α-methylbenzyl alcohol and 50 parts of toluene are preparedand mixed with varying amounts of steam. The vaporous steam-alcoholmixture is passed downward into a glass column (1 inch OD × 27 inchlength) having a 14 inch upper bed of silicon carbide (10 mesh)preheated to varying temperatures and a 6 inch lower bed of silica gel(same as in Example 2). Water and dehydrated organic product arecondensed, collected and separated as in Example 2. The results arerecorded in Table I.

To show the particular advantage of employing added water in thissystem, a control run (C₁) is made under conditions similar to the aboveruns with the exception that no water is added to the alcohol at anypoint prior to or during dehydration, the results of this control runare also recorded in Table I. To indicate upper limits as to temperatureduring dehydration, two control runs (C₂ and C₃) employing varyingamounts of water are also carried out in accordance with the proceduresemployed in the above sample runs. The results are recorded in Table I.

                                      TABLE I                                     __________________________________________________________________________    Sample      Reaction Impurities (2)                                           Run Water/Alcohol                                                                         Temperature (1),                                                                       Isopropenyl                                                                            4-(t-Butyl)Ethyl                                                                        Polymer (3)                           No. parts per part                                                                        ° C                                                                             Styrene, mole %                                                                        Benzene, mole %                                                                         Solubility                            __________________________________________________________________________    1   ˜5.0                                                                            410      0        0.15      Soluble                               2   ˜1.4                                                                            406      0        0.15      Soluble                               3   ˜0.84                                                                           406      0        0.14      Soluble                               4   ˜0.56                                                                           395      0        0.13      Soluble                               5   ˜20.0                                                                           393      0        0.2       Soluble                               6   ˜20.0                                                                           445      0        0.2       Soluble                               7   ˜20.0                                                                           505      <0.02    0.7       Soluble                               8   ˜0.50                                                                           400      0        0.1       Soluble                               9   ˜0.50                                                                           448      0        0.4       Soluble                               10  ˜0.50                                                                           506      <0.02    0.6       Soluble                               C.sub.1 *                                                                         ˜0                                                                              400      0.12     0.23      Insoluble                             C.sub.2 *                                                                         ˜20.0                                                                           550      >0.11    1.4       Insoluble                             C.sub.3 *                                                                         ˜0.50                                                                           550      >0.18    1.5       Insoluble                             __________________________________________________________________________     *Not an example of the invention                                              (1) Reaction temperature corresponds to temperature of silicon carbide        heat transfer means.                                                          (2) Approximate mole % of the specified impurity based on moles of            p-(t-butyl)styrene produced. Determined by Gas Phase Chromatography and       infrared Spectroscopy.                                                        (3) Solubility of 10% p-(t-butyl)styrene polymer in toluene at 23°     C.                                                                       

EXAMPLE 4

Several sample runs are carried out essentially according to Example 2except that a wide range of temperatures are employed. In the severalruns, mixtures of 50 parts of ar-(t-butyl)-α-methylbenzyl alcoholcontaining ˜7 mole percent of ar-(t-butyl)acetophenone and 50 parts oftoluene are prepared. A glass column (1 inch OD and 16 inch length)equipped with an electric furnace is filled to a height of 3.5 incheswith silicon carbide (8 mesh, 42 grams), to a total height of 13.5inches with silica gel (same as in Example 2, 50 grams), and to totalheight of 16.0 inches with silicon carbide (8 mesh, 40 grams) andpreheated to varying temperatures from 200° to 500° C. for the severalruns. Steam superheated to at least 550° C. and the alcohol/toluenemixture are added simultaneously into the feed end of the column atrates of 100 ml/hour (measured as condensed water) and 50 ml/hourrespectively. An intimate admixture of steam and the alcohol mixture invapor phase is formed and passes downward through the heat transferagent and the silica gel to effect dehydration. The water and organicproduct are then condensed, collected and separated. The organic productis distilled and dried, and its constituency is determined by infraredspectroscopy and vapor phase chromatography. The results are shown inTable II.

To point out the advantage of silica gel catalysts over conventionaldehydration catalysts, several control runs (C₄ -C₈) are made employingessentially the same procedure used above except that a titaniadehydration catalyst (4-8 mesh, and 70 m² g of surface area/g) issubstituted for silica gel. The dehydration column has a 3.5 inch bottomlayer of silicon carbide (8 mesh), a 10 inch middle layer of titaniacatalyst and a 2.5 inch top layer of silicon carbide. The organicproduct is recovered and analyzed by infrared spectroscopy and vaporphase chromatography and the results are recorded in Table II.

                                      TABLE II                                    __________________________________________________________________________                        Product Constituency, mole %                                    Reaction                              ar-(t-butyl)-                     Sample                                                                              Temperature,  ar-(t-butyl)-                                                                         ar-(t-butyl)-                                                                         ar-(t-butyl)-                                                                         α-methyl-                   Run No.                                                                             ° C                                                                           Catalyst                                                                             styrene ethyl benzene                                                                         acetophenone                                                                          benzyl alcohol                    __________________________________________________________________________    1     250°                                                                          Silica Gel                                                                           92.1    0.5     4.4     3.0                               2     300°                                                                          Silica Gel                                                                           95.3    0.3     4.5     --                                3     350°                                                                          Silica Gel                                                                           95.9    0.3     3.9     --                                4     400°                                                                          Silica Gel                                                                           97.2    0.2     2.6     --                                5     450°                                                                          Silica Gel                                                                           95.3    0.2     4.5     --                                6     500°                                                                          Silica Gel                                                                           94.2    0.5     5.3     --                                C.sub.4 *                                                                           250°                                                                          Anhydrous                                                                     Titania                                                                              97.3    0.4     2.3     --                                C.sub.5 *                                                                           300°                                                                          Anhydrous                                                                     Titania                                                                              97.2    0.6     2.2     --                                C.sub.6 *                                                                           350°                                                                          Anhydrous                                                                     Titania                                                                              94.1    1.9     4.0     --                                C.sub.7 *                                                                           400°                                                                          Anhydrous                                                                     Titania                                                                              91.9    5.4     2.7     --                                C.sub.8 *                                                                           450°                                                                          Anhydrous                                                                     Titania                                                                              83.5    12.3    3.3     --                                __________________________________________________________________________     *Not an example of the Invention                                         

As evidenced by Table II, significantly larger quantities ofar-(t-butyl)-ethyl benzene are generally produced in dehydrationemploying titania as catalyst than those employing silica gel underessentially the same conditions. The ar-(t-butyl)-ethyl benzene isdifficult to separate from ar-(t-butyl)styrene whereasar-(t-butyl)acetophenone is separated from either of the above by simpledistillation.

EXAMPLE 5

A solution of 50 parts of ar-(t-butyl)-α-methylbenzyl alcohol containing˜7 mole percent of ar-(t-butyl)acetophenone in 50 parts of toluene ismixed with superheated steam (550° C.) in a ratio of 2 parts of water toone part of the mixture. The steam-alcohol mixture is passed downwardthrough a glass column (1 inch OD × 21 inch length) containing a 10 inchupper layer of silicon carbide and a 10 inch lower layer of silica gel(same as in Example 2). The temperature at the top of the column is 350°C. and at the bottom of the column is 325° C. The water and organicproduct is distilled and dried, and its constituency is determined byinfrared spectroscopy and vapor phase chromatography. The results areshown in Table III.

For the purpose of comparison a control run (C₉) is carried out byfollowing the above process except that alumina (4-8 mesh and 210 m² gof surface area/gram) is substituted for silica gel as a dehydrationcatalyst. The organic product is distilled and dried and itsconstituency is determined by the means described above. The results arealso recorded in Table III.

                                      TABLE III                                   __________________________________________________________________________               Product Constituency, mole percent                                                                 ar-(t-butyl)toluene                                                                      ar-(t-butyl)acetophenone           Sample     p-(t-butyl)-                                                                         m-(t-butyl)-                                                                         ar-(t-butyl)                                                                         and        ar-(t-butyl)-α-methyl-       Run No.                                                                             Catalyst                                                                           styrene                                                                              styrene                                                                              ethylbenzene                                                                         ar-(t-butyl)benzene                                                                      benzyl alcohol,                    __________________________________________________________________________                                               ppm                                1     Silica                                                                             96.09  3.77   0.10   0.04       <25 ppm                                  Gel                                                                     C.sub.9 *                                                                           Alumina                                                                            96.52  2.32   1.08   0.08       ˜45 ppm                      __________________________________________________________________________     *Not an example of the Invention                                         

EXAMPLE 6

The dehydration process of the present invention is carried out in acontinuous manner by continuously feeding moltenar-(t-butyl)-α-methylbenzyl alcohol at 200 lb/hr and water superheatedto 550° C. at 400 lb/hr into a column (18 inch OD × 6 foot 8 inchlength). The column contains a 3 foot 4 inch upper bed of metallic heattransfer material preheated to 350° C. and a 3 foot 4 inch lower bed ofsilica gel (8-10 mesh, 340 m² /g, 140A average pore diameter). Thetemperature at the lower end of the column is 325° C. The dehydratedorganic product is continuously collected at the lower end of the columnand then recovered at 99⁺ percent purity by simple distillation. Thedehydrated product is determined by infrared spectroscopy to bear-(t-butyl)styrene.

EXAMPLE 7

Several samples of ar-chloro-α-methylbenzyl alcohol containing smallamounts of ar-chloro-acetophenone are continuously dehydrated by mixingthe liquid alcohol with varying amounts of superheated steam (550° C.)and passed as vapor phase through the column described in Example 3.Dehydration temperatures for the various runs are also varied. Theamounts of low boiling components are shown in Table IV.

For the purposes of comparison, similar samples ofar-chloro-α-methylstyrene also containing small amounts ofar-chloro-acetophenone are continuously dehydrated in the same mannerexcept that no water is added during the process. The amounts of lowboiling components for these control runs (C₁₀, C₁₁, C₁₂) are also shownin Table IV.

                                      TABLE IV                                    __________________________________________________________________________                             Low Boiling Components                                                        of Reaction Mixture,                                                          parts (1)         Low Boiling Impurities/            Sample                                                                              Water/Alcohol,                                                                         Reaction  ar-chloro-                                                                          ar-chloro-  ar-chlorostyrene,                  Run No.                                                                             ml/100 ml                                                                              Temperature, ° C                                                                 styrene                                              ethylbenzene                                                                        other    parts/100 parts                                                __________________________________________________________________________    1     3        400       62.89 1.10    0.22                                                                              ˜2.1                         2     5        400       76.19 0.88    0.19                                                                              ˜1.4                         C*.sub.10                                                                           0        400       55.80 1.88    0.45                                                                              ˜4.2                         3     25       350       73.91 0.16    0.09                                                                              ˜0.34                        C*.sub.11                                                                           0        350       58.90 0.89    0.27                                                                              ˜1.80                        5     50       400       78.11 0.03    ND**                                                                              ˜0.04                        C*.sub.12                                                                           0        400       63.64 1.61    0.45                                                                              ˜3.23                        __________________________________________________________________________     *Not an example of the invention                                              **None Detected                                                               (1) Higher Boiling Components including chloroacetophenone and                ar-chloro-α-methylbenzyl alcohol comprise the remaining reaction        mixture having a total of 100 parts.                                     

EXAMPLES 8-13

In accordance with the continuous dehydration process of Example 3,several substituted α-methylbenzyl alcohols are dehydrated to thecorresponding substituted styrenes thereof. The results obtained arecomparable to those obtained in Example 3. The alcohols successfullydehydrated are as follows:

ar-t-butyl-α,α-dimethylbenzyl alcohol

ar-dichloro-α-methylbenzyl alcohol

ar-dibromo-α-methylbenzyl alcohol

ar-di-t-butyl-α-methylbenzyl alcohol

ar-(1-ethyl-1-methylpentyl)-α-methylbenzyl alcohol

ar-t-butyl-ar-melthyl-α-methylbenzyl alcohol.

Several dehydration runs are also carried out using silica gel catalystshaving different mesh sizes in the range from about 2 to about 400 andsurface areas in the range from about 300 to about 900 m² /g with goodresults.

What is claimed is:
 1. A process for preparing an ar-(t-alkyl)styrene bydehydrating an α-alkylbenzyl alcohol represented by the general formula##STR2## wherein each R is independently hydrogen, alkyl having from 1to 12 carbon atoms or halogen provided that an R group is t-alkyl and R₁is hydrogen or alkyl having 1 to 4 carbon atoms which method comprises adehydration step of contacting the α-alkylbenzyl alcohol in vapor phasewith a silica gel dehydration catalyst consisting essentially of silicagel in the presence of from about 0.03 to about 25 parts by weight ofadded water per part by weight of alcohol, said dehydration step beingcarried out at temperatures above 260° up to about 510° C., andsubsequent simple distillation step of distilling the dehydrationproduct to obtain the ar-(t-alkyl)styrene in purity greater than about99 mole percent and containing less than 0.02 mole percent of diolefinicimpurity.
 2. The process according to claim 1 wherein the water in theform of steam is mixed with the α-alkylbenzyl alcohol prior tocontacting the alcohol with silica gel.
 3. The process according toclaim 1 wherein the silica gel is in the form of a divided solid havinga surface area of at least about 300 square meters/gram.
 4. The processaccording to claim 3 wherein the silica gel is divided, porous solidhaving an average pore diameter ranging from about 2 to about 200Angstrom units.
 5. the process according to claim 1 wherein the alcoholis ar-chloro-ar-(t-butyl)-α-methylbenzyl alcohol.
 6. The processaccording to claim 1 wherein the alcohol isar-bromo-ar-(t-alkyl)-α-methylbenzyl alcohol.
 7. The process accordingto claim 1 wherein the alcohol is an ar-(t-butyl)-α-methylbenzylalcohol.
 8. The process according to claim 7 wherein the t-alkyl ist-butyl.
 9. The process according to claim 7 wherein the t-alkyl ist-amyl.
 10. A continuous process for preparing ar-(t-alkyl)styrene whichcomprises (1) passing a vapor phase of an ar-(t-alkyl)-α-methylbenzylalcohol and from about 0.5 to about 20 parts by weight of water in theform of superheated steam per part of the alcohol through a columncontaining a heat transfer agent and a silica gel dehydration catalystconsisting essentially of silica gel in the form of a divided solidhaving a surface area of at least 300 square meters per gram, saidprocess being carried out at a dehydration temperature from about 260°to about 425° C. such that the alcohol is dehydrated, and (2) recoveringby simple distillation the resulting dehydration product in the form ofar-(t-alkyl)styrene of purity greater than about 99 mole percent andcontaining less than 0.02 mole percent of diolefinic impurity.
 11. Theprocess of claim 1 wherein the alcohol is ar-(t-butyl)-α-methylbenzylalcohol consisting essentially of the meta and para isomers thereof,from about 0.5 to about 20 parts by weight of water per part of alcoholare employed and the temperature is within the range from 390° to 505°C.
 12. The process of claim 10 wherein the dehydration temperature is inthe range from about 325° to about 425° C.